Reading Files - Beginning Java 2 SDK - page 427

The third possibility is to use a FileDescriptor object that you have obtained by calling the
getFD() method for an existing FileInputStream object, or possibly a RandomAccessFile
object. For example:
File aFile = new File("C:/Beg Java Stuff/myFile.text");
FileInputStream inputFile1 = null; // Place to store an input stream reference
FileDescriptor fd = null; // Place to store the file descriptor
try {
// Create the stream opened to write
inputFile1 = new FileInputStream(aFile);
fd = inputFile1.getFD(); // Get the file descriptor for the file
} catch(IOException e) { // For IOException or FileNotFoundException
e.printStackTrace(System.err);
System.exit(1);
}
// We can now create another input stream for the file from the file descriptor...
FileInputStream inputFile2 = new FileInputStream(fd);
The getFD() method can throw an exception of type IOException if an I/O error occurs. Since
IOException is a superclass of FileNotFoundException , the catch block will catch either type
of exception.
I'm sure that you noticed that the constructor call that creates inputFile2 is not in a try block. This is not
an oversight. When you create the FileInputStream object from a FileDescriptor object, the
FileNotFoundException cannot be thrown since a FileDescriptor object always refers to an existing
file. This is because a FileDescriptor object can only be obtained from a file stream object that already
encapsulates a connection to a physical file so there can be no doubt that the file is real.
As with the FileOutputStream constructors, any of the FileInputStream constructors will throw
a SecurityException if a security manager exists on the system and read access to the file is not
permitted. Since this is a type of RuntimeException you don't have to catch it.
File Channel Read Operations
You obtain a reference to a FileChannel object that you can use to read a file by calling the
getChannel() method of a FileInputStream object. Since a FileInputStream object opens a
file as read-only, only channel read operations are legal. The channel returned by a
FileInputStream object has three basic read operations available, each of which read starting at the
byte indicated by the current position in the file. The file position will be incremented by the number of
bytes read. The three read() methods for a FileChannel object are: